Low molecular weight polymers among water-soluble polymers such as (meth)acrylic acid-based polymers are excellent in chelating ability and dispersibility, and therefore, they have been suitably used as such additives as dispersants for inorganic pigments, metal ions, and the like, or scale inhibitors. The scale inhibitors, for example, are often used in cooling water systems where highly concentrated water is used so as to save resource and water, high-hardness water systems for decrease in water quality, or water systems with high salt concentration water such as sea water. However, in such water systems, the polymers disadvantageously gel and precipitate, resulting in a significant decrease in the scale inhibiting ability.
Patent Literature 1 discloses a (meth)acrylic acid-based copolymer which comprises a structural unit derived from a (meth)acrylic acid-based monomer represented by Formula (R1) below and a structural unit derived from a monoethylenic unsaturated monomer copolymerizable with the (meth)acrylic acid-based monomer. The copolymer includes a main chain that has a sulfonic acid group (which may be in the form of an ammonium salt, an alkali metal salt, or an organic amine group salt) at at least one end thereof. The structural unit derived from a monoethylenic unsaturated monomer at least comprises a structural unit derived from a (meth)allyl ether-based monomer represented by Formula (R2) below. The proportion of the structural unit derived from a (meth)acrylic acid-based monomer is 70 to 95 mol %, and that of the structural unit derived from a (meth)allyl ether based-monomer is 5 to 30 mol %. The copolymer has a gel resistance determined by a gel resistance test in a calcium ion-containing water system of 100 to 1500.

In Formula (R1), R1 represents a hydrogen atom or a methyl group, and X represents a hydrogen atom, a metal atom, an ammonium group, or an organic amine group.

In Formula (R2), R2 represents a hydrogen atom or a methyl group, and Y and Z each independently represent a hydroxy group or a sulfonic acid group (which may be in the form of a monovalent metal salt, a divalent metal salt, an ammonium salt, or an organic amine group salt).
Patent Literature 1 describes that the above copolymer has a significantly improved gel resistance owing to a synergistic effect produced by the sulfonic acid group present in a side chain that is a structural unit derived from a (meth)allyl ether based-monomer and the sulfonic acid group introduced to the end of the main chain.